The present invention relates generally to a miniature drive unit and, more particularly, to a rotor for a miniature electric motor incorporated in, e.g., an electronic clock or watch.
An electronic clock or watch, such as a quartz watch, is provided with a miniature precision electric motor as a drive unit. The electric motor, generally structured as a stepping motor, used in the electronic clock includes a rotor with an annular or cylindrical permanent magnet and a stator with a coil. The rotor is also provided with a shaft concentric with and secured to the cylindrical permanent magnet, and a driving toothed wheel generally integrally formed on the shaft.
In the rotor of a conventional miniature electric motor incorporated in, e.g., an electronic clock, a rotor shaft is generally fixed in a center through hole formed in a rotor magnet by using a bonding material, such as an adhesive or solder, or by tightly press-fitting the shaft into the through hole. In this fixing operation, it is required to maintain a concentricity or alignment between the shaft and the annular or cylindrical rotor magnet, and to ensure a large and stable fixing force for securing the shaft in a predetermined proper position in the magnet. When the rotor shaft is tightly press-fitted into the through hole of the rotor magnet, it is also required to prevent the magnet from being broken or cracked due to the stress concentration, in a region adjacent to the through hole of the magnet, during the pressure-fitting operation of the shaft and for a long time after the rotor is completely assembled.
One example of the rotor of the conventional miniature motor in the electronic clock is disclosed in Japanese Unexamined Utility Model Publication (Kokai) No. 54-71610 (JP-U-54-71610). This motor rotor includes a shaft with a toothed wheel and a cylindrical permanent magnet with a center through hole for fixing the shaft. The rotor shaft is provided with a deformed profile such as a prism, and is tightly press-fitted into the cylindrical center through hole of the rotor magnet. In this structure, the shaft is in a local contact with the inner cylindrical surface of the magnet in the through hole at a part of the outer surface of the shaft, over the entire axial length of the through hole. This local contact of the shaft with the magnet serves to distribute a stress concentration into some local regions adjacent to the through hole of the magnet, which can prevent the magnet from being broken or cracked and also can ensure a large fixing force.
The annular or cylindrical permanent magnet of the conventional miniature motor rotor is generally formed from a sintered magnet made from rare-earth elements. A sintered permanent magnet normally has high magnetic performance, but tends to increase a production cost of a small, annular or cylindrical magnet structure. On the other hand, a permanent magnet with a simply molded structure, generally referred to as xe2x80x9ca bonded magnetxe2x80x9d, is known to be capable of reducing such a production cost. However, when the annular or cylindrical magnet of the miniature motor rotor is formed from a bonded magnet, the mechanical strength of the rotor magnet is reduced in comparison with that of the sintered rotor magnet. Therefore, in this case, even if a rotor shaft has a structure as being described in JP-U-54-71610, the distributed stress concentration may cause the bonded rotor magnet to be broken or cracked during the press-fitting operation of the shaft or after the rotor is completed.
JP-U-54-71610 also discloses a bush interposed between the rotor shaft and the rotor magnet in the through hole, the bush being used to prevent the magnet from being broken or cracked. Such a bush is also disclosed in Japanese Unexamined Utility Model Publication (Kokai) No. 56-71078 (JP-U-56-71078). However, the additional use of the bush increases the number of parts of the motor rotor, and may deteriorate the productivity of the latter and increase the production cost thereof.
When the rotor shaft is fixed to the through hole of the rotor magnet by a bonding material such as an adhesive or solder, the shaft can be loosely fitted into the through hole, whereby the problems of breaking or cracking the magnet can be eliminated and relatively large fixing force can be established. However, a clearance between the shaft and the magnet in the through hole is normally very small in the order of several xcexcm, so that the bonding material may overflow from the through hole as the shaft is inserted into the latter. Therefore, in this solution, it is necessary to carefully and precisely apply the bonding material to the very small clearance between the shaft and the magnet so as to ensure the large and stable fixing force, which may prevent mass production of the miniature motor rotor.
It is therefore an object of the present invention to provide a rotor, used in a miniature electric motor, which can be produced with a high structural reliability, a high yield and a relatively low cost.
It is another object of the present invention to provide a rotor including an annular or cylindrical magnet and a shaft fixed to the magnet, which can prevent the magnet from being broken or cracked due to the shaft, and can ensure a large and stable fixing force for securing the shaft in a predetermined position on the magnet.
It is further object of the present invention to provide a fixing structure, for fixing a shaft to an annular or cylindrical magnet, which can be suitably adopted for a process for producing a rotor having a relatively fragile magnet, such as a simply molded or bonded magnet.
In accordance with the present invention, there is provided a rotor, for an electric motor, comprising a magnet having a rotation axis, the magnet being provided with a through hole extending coaxially with the rotation axis; a shaft fixed concentrically to the magnet, the shaft including a portion fitted in the through hole, the portion having an axial interengagement length shorter than an axial length of the through hole; and reinforcing means provided at least inside the through hole for ensuring a fixing force to securely hold the shaft in a predetermined position in the magnet.
In a preferred aspect of the invention, the magnet comprises an annular magnet material and a coating formed on a surface of the magnet material and arranged at least inside the through hole, and the reinforcing means comprises the coating, the portion of the shaft being engaged with the coating in a face-to-face manner.
In this arrangement, it is preferred that the coating is made of a metal plating.
The metal plating may be an electroless plating.
Also, the metal plating may include at least one of a Nixe2x80x94P electroless plating, a Nixe2x80x94B electroless plating and a Nixe2x80x94Pxe2x80x94W electroless plating.
It is preferred that the metal plating has a thickness of at least 10 xcexcm.
Alternatively, the metal plating may include an electroless plating base layer and an electroplating top layer.
In this arrangement, the electroplating top layer may be a Ni electroplating.
The electroless plating base layer may have a thickness in a range of 0.5 xcexcm to 2.0 xcexcm.
Also, the electroplating top layer may have a thickness of at least 3.0 xcexcm.
Preferably, the magnet material is made of a bonded magnet material.
In this arrangement, a dimensional relationship between the axial interengagement length of the portion of the shaft and the axial length of the through hole may be defined as T/5xe2x89xa6txe2x89xa6T/2, in which xe2x80x9cTxe2x80x9d is the through hole axial length and xe2x80x9ctxe2x80x9d is the axial interengagement length.
Also, it is advantageous that, in an arrangement that the portion of the shaft is tightly press-fitted in the through hole of the magnet, an interference of the portion in the through hole is in a range of 5 xcexcm to 30 xcexcm.
Preferably, the bonded magnet material is vacuum-impregnated with a bonding agent or filler.
In another preferred aspect of the invention, the reinforcing means comprises an adhesive filled in a clearance defined between a reminder of the shaft other than the portion and the magnet inside the through hole.
In this arrangement, a dimensional relationship between the axial interengagement length of the portion of the shaft and the axial length of the through hole may be defined as T/5xe2x89xa6txe2x89xa64T/5, in which xe2x80x9cTxe2x80x9d is the through hole axial length and xe2x80x9ctxe2x80x9d is the axial interengagement length.
The adhesive may be a thermosetting epoxy resin.
The magnet may comprise an annular magnet material and a coating formed on a surface of the magnet material at least inside the through hole, and the reinforcing means may further comprise the coating, the portion of the shaft being engaged with the coating in a face-to-face manner.
In this arrangement, the coating may be made of a metal plating.
Alternatively, the coating may be made of an organic substance layer.
In this arrangement, it is preferred that the magnet material is made of a bonded magnet material.
The magnet may contain rare-earth elements.
In accordance with the present invention, there is also provided a method of producing a rotor for an electric motor, comprising the steps of (a) forming a coating on a surface of an annular magnet material and thereby providing a magnet having a rotation axis and a through hole extending coaxially with the rotation axis, the coating being arranged at least inside the through hole; (b) providing a shaft including a portion capable of being fitted in the through hole; and (c) inserting the shaft into the through hole of the magnet and tightly press-fitting the portion of the shaft in the through hole, until an axial interengagement length of the portion, shorter than an axial length of the through hole, is obtained.
In a preferred aspect of the invention, the magnet material may be made of a bonded magnet material, the coating may be a metal plating, and the method may further include a step of vacuum-impregnating the bonded magnet material with an adhesive before the step of forming the coating.
An interference of the portion in the through hole may be adjusted by changing a thickness of the coating.
The present invention further provides a method of producing a rotor, for an electric motor, comprising the steps of (a) providing a magnet having a rotation axis and a through hole extending coaxially with the rotation axis; (b) providing a shaft including a first portion capable of being fitted in the through hole and a second portion axially adjacent to the first portion for defining a clearance inside the through hole; (c) inserting the shaft into the through hole of the magnet and fitting the first portion of the shaft in the through hole, until an axial interengagement length of the first portion, shorter than an axial length of the through hole, is obtained; and (d) filling an adhesive in the clearance inside the through hole.
In a preferred aspect of the invention, the adhesive may be vacuum-impregnated into the clearance.
The magnet may be made of an annular bonded magnet material, and the method may further include a step of forming a coating on a surface of the bonded magnet material before the step of inserting the shaft.